1. Field of the Invention
The present invention relates to current excitation type light-emitting elements. Further, the present invention relates to light-emitting devices and electronic devices including the light-emitting elements.
2. Description of the Related Art
In recent years, research and development have been actively conducted on light-emitting elements utilizing electroluminescence. As a basic structure of these light-emitting elements, a structure in which a light-emitting substance is interposed between a pair of electrodes is used. By applying voltage to these elements, light emission can be obtained from the light-emitting substance.
Compared to liquid crystal displays, such light-emitting elements are of self-light-emitting type and have advantages such as high visibility of pixels and no need of backlights. Thus, the light-emitting elements are suitable for flat panel display elements. In addition, such light-emitting elements can be manufactured to be thin and lightweight, which is also highly advantageous. Further, the light-emitting elements have very high response speed.
Since the light-emitting elements can be formed into a film shape, surface light emission can be easily obtained by forming large-area elements. This characteristic is not easily obtained in point light sources typified by incandescent lamps or LEDs, or line light sources typified by fluorescent lamps. Therefore, the above described light-emitting elements also have a high utility value as a surface light source which is applicable to lighting systems and the like.
Light-emitting elements utilizing electroluminescence are broadly classified into two types according to whether the light-emitting substance is an organic compound or an inorganic compound.
In the case where the organic compound is used as a light-emitting substance, by voltage application to a light-emitting element, electrons and holes are injected to a layer containing a light-emitting organic compound from a pair of electrodes, so that current flows through the light-emitting element. Then, recombination of these carriers (the electrons and holes) makes the light-emitting organic compound be in an excited state and emit light when the excited state returns to a ground state. Based on this mechanism, such a light-emitting element is referred to as a current excitation type light-emitting element.
Note that excited states of the organic compound include a singlet excited state and a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
Such light-emitting elements have many problems on materials, in improving element characteristics. In order to overcome the problems, improvement in element structure, development of materials, and the like have been carried out.
For example, in Reference 1 (Tetsuo Tsutsui and eight others, Japanese Journal of Applied Physics, vol. 38, L1502-L1504, 1999), a hole-blocking layer is provided in a light-emitting element which uses a phosphorescent material, so that the light-emitting element emits light efficiently.